1. Field of the Invention
The present invention relates generally to electro-optical reconnaissance systems, and more particularly, to non-linear scanning to optimize the performance of electro-optical reconnaissance systems.
2. Related Art
Electro-optical systems enjoy widespread use in contemporary reconnaissance systems. There are three primary reasons for this popularity. The first reason is that these systems are able to operate in real time. In other words, these systems are able to process and interpret dam as it is collected. These systems collect data using an airborne camera system, transmit the data to a ground station via an air-to-ground data link, and process the data at the ground station as it is received. This allows data to be interpreted much more quickly than similar data recorded on photographic film, flown back to a home base, and processed subsequent to the flight operation.
A second advantage is the ability of the electro-optical system to penetrate haze. This ability is made possible by signal processing techniques which are able to separate and enhance the data information from background noise (haze). This ability does not exist with conventional photographic reconnaissance techniques since it is not possible to remove the effects of background noise.
Additionally, because of the high sensitivity of electro-optical detectors, electro-optical systems can operate with less ambient light than photographic systems. This has the effect of extending the amount of time per day during which a reconnaissance mission can be flown.
There are typically two general forms of electro-optical reconnaissance systems. FIG. 1 illustrates the two general forms of electro-optical reconnaissance systems. In a first mode, called a strip mode system, the area detected by the electro-optical system is a long, narrow slit which can be described as a projection of a slit 104. The projection of slit 104 is the area detected (projected) by a focal plane array (FPA) of the system. The FPA is mounted in an aircraft 102. A lens arrangement is used to focus slit 104 onto the FPA. Typically, the FPA is a line of optical sensor devices such as CCDs. In a strip mode system, the projection of the slit 104 extends at right angles to the direction of flight and constitutes one dimension of the image. The direction of flight is shown by a flight path 122. The second dimension of the image is generated by the forward motion of aircraft 102 as it flies along flight path 122 at a velocity V.
In this specification, the direction of forward motion of the aircraft will be referenced as the in-track direction. The direction at a right angle to the flight path is referred to as the cross-track direction.
The second mode is a sector scan panoramic mode (sector scan mode). In the sector scan mode, the line of detectors in the FPA is aligned in the in-track direction. Hence, a projection 106 of the FPA is in the in-track direction. Projection 106 is scanned at a right angle to the flight path (the cross-track direction) across the scene to be imaged. Scanning in the cross-track direction provides the second dimension of the image.
For long-range aerial surveillance applications, long-range oblique photography (LOROP) systems are utilized. A typical LOROP system uses an aircraft-mounted electro-optical camera configured to scan a scene at or near the horizon in the sector-scan mode. The scanned objects are focused by a lens or other optics onto the FPA. A rotating prism may be used to scan projection 106 in the cross-track direction across the scene to be sampled. The FPA is often one picture element (pixel) high and several thousand pixels wide.
The electro-optical camera generates an electronic signal that represents an image of the scene scanned. This signal is downlinked to a ground station where it is converted into visual information.
Referring again to FIG. 1, a LOROP system will be described in more detail. An airplane 102 flies at an altitude A above the ground and at a ground distance D from the scene to be photographed. Airplane 102 travels at velocity V in the in-track direction parallel to the scene. The line-of-sight distance between airplane 102 and the scene is defined as a slant range .rho..sub.slant. For long-range (LOROP) applications, slant range .rho..sub.slant is large. (For example, in a typical application, .rho..sub.slant can be on the order of 40 nautical miles).
The FPA and associated optics are mounted in airplane 102. A depression angle .theta..sub.d is defined as the angle of the camera's line-of-sight with respect to a horizontal plane. A rotating camera barrel causes projection 106 to be scanned across the scene at or near the horizon. Velocity V of airplane 102 in the in-track direction and parallel to the scene causes the camera to photograph adjacent slices of the scene. Each adjacent slice forms a complete picture.
FIG. 2 illustrates these scanned slices in more detail. Referring to FIGS. 1 and 2, the length of each slice is determined by the distance covered by the scanning motion of the camera in the cross-track direction. This length is referred to as a cross-track field-of-coverage 202. The width of each slice in the in-track direction is defined by the focal plane array width, the focal length of the optics, and the distance between the camera and the scene. This width is known as the in-track field-of-coverage 204. The slices overlap each other in the in-track direction by an amount known as a forward overlap 206. Forward overlap 206 ensures that no part of the scene is left unscanned.
In-track field-of-coverage 204 is a function of the .rho..sub.slant. According to lens arrangements typically employed, in-track field-of-coverage 204 is larger (larger on the ground, but the same angular coverage) at the far end of the scan (far-field point of scan) than it is at the point of scan closest to the aircraft (near-field point of scan). This phenomenon is not illustrated in FIG. 2 for simplicity. Instead, FIG. 2 illustrates an in-track field-of-coverage 204 as the same for both the near-field and the far-field point of scan.
A vertical scan velocity (in the cross-track direction) is selected so that for a given airplane 102 velocity V, a specified amount of forward overlap 206 is obtained. The amount of forward overlap 206 specified is chosen so that no image information is missed between scans. As the velocity V of airplane 102 increases, the vertical scan velocity must also increase to maintain the specified amount of forward overlap 206.
To obtain optimum system resolution, the information in the FPA must be read each time the vertical scan causes the FPA to traverse the area projected by each pixel. As more area is detected (projected) by a pixel between FPA reads (i.e., as scanning velocity increases), system resolution diminishes. Thus, to maintain system resolution, as the vertical scan velocity increases, the rate at which the information in the FPA is read must increase as well. Because the rate a which the information in the FPA may be read is limited by detector technologies, the vertical scan velocity is limited to a practical maximum rate. Since vertical scan velocity is bounded to a practical maximum by the maximum FPA read rate, and since an increase in vertical scan velocity is required to maintain a specified forward overlap 206 as airplane 102 velocity V increases, velocity V is limited to a maximum. Thus, for a given resolution and forward overlap, the operational velocity V at which sector-scan panoramic reconnaissance can be performed is limited to a maximum. Conventional LOROP systems have not been able to overcome this limitation without sacrificing resolution.
Note that in this document, system resolution is defined in terms of some constant number of line pairs per unit length on the ground.
What is needed is a system and method for increasing the operational velocity V at which an aircraft performing electro-optical reconnaissance can fly while maintaining a specified forward overlap 206, and maintaining system resolution to a given level.